The present invention relates to an electronic scanning antenna, and more particularly to an electronic scanning antenna for scanning a pencilbeam within a range of a broad elevation angle used in a radar system. Specifically, the present invention is concerned with an improvement in aperture efficiency and radiation characteristics in an electronic scanning antenna applicable to a broad scanning angle range.
In general, electronic scanning antennas functioning to scan a radiation beam (inclusive of a single beam and a multi-beam) based on a phase electronic scanning system so as to correspond to a predetermined broad scanning angle range have been widely used in a radar system etc. In a commonly used electronic scanning antenna in which a radiation beam is scanned in a vertical plane, for instance, with a view to enlargement of a scanning angle range without markedly impairing the radiation characteristics of the radiation beam, a radiation aperture unit 1 is installed so that an elevation angle of a normal 101 of the aperture unit 1 becomes equal to .theta..sub.N with respect to a horizontal line 102 as shown in FIG. 1. For example, as shown in this figure, the radiation aperture unit 1 is set so that a radiation beam 103 is scanned over a broad elevation angle range from the horizontal line 102 to an elevation angle .theta..sub.S. In general, as well known in the art, a radiation beam which is radiated from a radiation aperture unit has its radiation characteristics restricted by an electric field distribution at the radiation aperture unit. Also, to increase an aperture efficiency of the radiation aperture unit and reduce a sidelobe level to improve the radiation characteristics, the essential requirement is to set a phase distribution in the electric field distribution to a predetermined in-phase state and to set an amplitude distribution in the electric field distribution to a predetermined state. In the case shown in FIG. 1, even in the condition where an electric field distribution at the radiation aperture unit 1 is suitably set in a manner stated above, the beam width in a vertical plane of the radiating beam 103 formed in a direction of an elevation angle .theta. is approximately proportional to 1/cos (.theta.-.theta..sub.N). Accordingly, the beam width in the horizontal direction 102 which requires the narrowest beam width in a radar system is expanded by a multiple of the order of 1/cos .theta..sub.N as compared to the beam width in the direction of the elevation angle .theta..sub.N. In addition, when the elevation angle .theta..sub.N is relatively large, there is a tendency that the sidelobe level increases, resulting in such unfavorable phenomena that both reduced aperture efficiency and degraded radiation characteristics are concurrently caused. In principle, as far as detection capability of the radar system is concerned, it is strongly required to increase the aperture efficienty and improve the radiation characteristics etc., in obtaining antenna functions within a small elevation angle range about the horizontal line. Nevertheless, as the scanning angle range of a radiation beam increases, there still arises the above problem that the antenna function is degraded within a small elevation angle range about the horizontal line.
To eliminate the above-mentioned drawback, another example of an electronic scanning antenna as shown in a conceptual block diagram of FIG. 2 has been proposed wherein a radiating beam is scanned in a vertical plane similar to the electronic scanning antenna shown in FIG. 1. An antenna radiation section comprises two radiation aperture units 2 and 3 which are positioned so that normal 104 and 105 to respective radiation aperture surfaces form elevation angles of .theta..sub.N1 and .theta..sub.N2 with respect to a horizontal line 106. A radiation beam radiated from the radiation aperture unit 2 is scanned over an angular range of elevation angles from 0 (zero) to .theta..sub.S1. On the other hand, a radiation beam which is radiated from the radiation aperture unit 3 is scanned over an angular range of elevation angle from .theta..sub.S1 to (.THETA..sub.S1 +.theta..sub.S2). During transmission, a transmission signal from a terminal 51 is inputted to an RF power switch 4. The RF power switch 4 effects switching operation under the control of a radiation beam control signal from a terminal 52 to feed power to either the radiation aperture unit 2 or the radiation aperture unit 3. Accordingly, the radiation beam is scanned over an angular range from elevation angles 0 (zero) to (.theta..sub.S1 +.theta..sub.S2), on the basis of the radiation beam scanning function of the radiation aperture units 2 and 3 and the signal switching fucntion of the RF power switch 4. In general, the antenna configuration is in no way limited to the case where the antenna radiation section comprise only two radiation aperture units as shown in FIG. 2. For instance, the antenna radiation section may comprise a plurality of, more than two, radiation aperture units. Further, among the radiation aperture units as constituent elements of the antenna radiation section, there may exist one or more radiation aperture units without provision of a radiation beam scanning function.
In the prior art electronic scanning antenna shown in FIG. 2, the antenna radiation section is provided with two radiation aperture units 2 and 3, thereby restricting the scanning angle range for a radiation beam at each radiation aperture unit to a relatively narrow angular range. Thus, this can suppress the degradation of the aperture efficiency including broadening of a radiation beam at the time of beam scanning etc. and the degradation of the radiation characteristics, as compared to the electronic scanning antenna shown in FIG. 1.
However, in the electronic scanning antenna shown in FIG. 2, a radiation beam within a small elevation angle range about the horizontal line 106 is formed only by the radiation aperture unit 2, and the radiation aperture unit 3 does not at all contribute to the formation of this radiation beam. Accordingly, the radiation aperture units 2 and 3 which should effectively function as an antenna radiation section merely become active as a partially limited aperture within a small elevation angular where detection capability of radar sytems should be quaranteed, resulting in the drawback that the degree of the aperture efficiency is not sufficient for the antenna function.